1. Field of the Invention
The present invention is related to a radiation image detection device for use in a radiation imaging system and a method for controlling the same.
2. Description Related to the Prior Art
A radiation imaging system, for example, an X-ray imaging system is composed of an X-ray generating apparatus and an X-ray imaging apparatus. The X-ray generating apparatus generates X-rays. The X-ray imaging apparatus captures an X-ray image formed by the X-rays passed through a subject. The X-ray generating apparatus has an X-ray source, a source control device, and an irradiation switch, for example. The X-ray source emits the X-rays to a subject (a patient). The source control device controls the X-ray source. The irradiation switch is used for inputting a command to start X-ray irradiation. The X-ray imaging apparatus has an X-ray image detection device and an imaging control device. The X-ray image detection device detects the X-ray image based on the X-rays passed through the subject. The imaging control device controls the X-ray image detection device.
For the X-ray image detection device, recently, a flat panel detector (FPD) has been commonly used as an X-ray image detector instead of an X-ray film or an imaging plate (IP). The FPD has pixels arranged in a matrix. Each pixel accumulates signal charge in accordance with an amount of the incident X-rays. By accumulating the signal charge on a pixel-by-pixel basis, the FPD detects an X-ray image representing image information of the subject. Then, the FPD outputs the X-ray image as digital image data.
A portable X-ray image detection device (hereinafter referred to as the electronic cassette) has also been put to practical use. The electronic cassette has the FPD enclosed in a rectangular solid housing. The electronic cassette may be attached to an imaging bed or platform originally designed for a film cassette or an IP cassette. Alternatively, for example, when it is difficult to capture an image of a region of interest using a stationary imaging system because of the location of the region of interest, the patient himself or herself may hold the electronic cassette when the X-ray imaging is performed. The electronic cassette may be used on-site (outside of a hospital) without the use of the imaging bed, to perform X-ray imaging of a patient requiring emergency medical care in, for example, an accident or disaster scene or of an elderly patient requiring home care.
Conventionally, to synchronize the X-ray irradiation caused by pressing of the irradiation switch and the start of the accumulation operation of the signal charge by the X-ray image detection device, the source control device of the X-ray generating apparatus and the imaging control device of the X-ray imaging apparatus exchange an operation signal, being a synchronization signal representing the start of the X-ray irradiation, issued by the irradiation switch. In this case, the X-ray generating apparatus and the X-ray imaging apparatus need to be connected to each other electrically. However, if the X-ray generating apparatus and the X-ray imaging apparatus are produced by different manufacturers and their connection interfaces (for example, standards of cables, connectors, and formats of the synchronization signals) are not compatible with each other, a new interface needs to be provided.
To solve the problem, it is suggested that the X-ray image detection device itself detects the start of the irradiation of the X-rays to synchronize with the X-ray generating apparatus without exchanging the synchronization signal (without the electrical connection) between the X-ray image detection device and the X-ray generating apparatus (see U.S. Patent Application Publication No. 2003/0086523 corresponding to Japanese Patent Laid-Open Publication No. 2003-126072, U.S. Pat. No. 6,797,960 corresponding to Japanese translation of PCT International Publication No. 2002-543684, and U.S. Patent Application Publication No. 2010/0054405 corresponding to Japanese Patent Laid-Open Publication No. 2008-125903).
In the U.S. Patent Application Publication No. 2003/0086523, the X-ray image detection device performs the read-out operation (nondestructive read-out) of the signal charge at a predetermined frame rate. A difference between an image of the present frame outputted by the read-out operation and an image of the previous frame is obtained. The difference is compared with a threshold value. When the difference exceeds the threshold value, it is detected or judged that the X-ray irradiation has been started. In the U.S. Pat. No. 6,797,960, the bias current of the FPD is detected. The bias current is an output value from an area of the FPD where the X-rays are incident without passing through a subject. A differential value of the bias current is compared with a threshold value to detect or judge the start of the X-ray irradiation. In the U.S. Patent Application Publication No. 2010/0054405, the X-ray image detection device is provided with a photodiode for detecting the X-rays. A reference signal generated based on an offset value of the photodiode is compared with an output signal from the photodiode. When the output signal value exceeds the reference signal value, it is determined that the X-ray irradiation has been started.
Generally, an output from an electrical component is affected by noise caused by an internal factor of the component itself or an external factor such as ambient environment. The X-ray image detection device composed of a plurality of electrical components is no exception. For example, when a subject or radiological technologist bumps into the X-ray image detection device inadvertently, vibration or shake caused by the impact causes noise. False detection of the X-ray irradiation occurs if the noise affects a signal for detecting the start of the X-ray irradiation, though the X-ray irradiation has not been started actually. Power consumption increases when the false detection induces unnecessary operations of the X-ray image detection device. Because the X-ray imaging cannot be performed during the irradiation detection operation, an operator is likely to miss the timing for shooting.
Moreover, the imaging control device and a device (for example, a console) for setting imaging conditions, both connected to the X-ray image detection device, may go through unnecessary operations as if the image capture has been performed. Accordingly, burdensome operations such as resetting the imaging conditions are necessary. Even if the start of the X-ray irradiation is detected or judged erroneously, it may be handled as if the X-ray imaging is performed appropriately. An image resulting from the false detection may be transmitted as a correct image to a doctor. This may cause medical malpractice.
In the U.S. Patent Application Publication No. 2003/0086523, a difference between the image of the present frame and the image of the previous frame is compared with the threshold value. Noise is added only to the present frame, and thereby the image signal exceeds the threshold value. As a result, the false detection occurs. In the method of the U.S. Pat. No. 6,797,960, there is a possibility of the false detection when the bias current varies due to noise. Similarly, in the method of the U.S. Patent Application Publication No. 2010/0054405, the false detection occurs when a noise component greater than or equal to the reference signal is added to the output from the photodiode.
All of the methods described in the U.S. Patent Application Publication No. 2003/0086523, the U.S. Pat. No. 6,797,960, and the U.S. Patent Application Publication No. 2010/0054405 are susceptible to noise, so these methods are at increased risk of erroneously detecting the start of the X-ray irradiation.
However, none of the above describes how to prevent the false detection caused by noise.